This invention relates to methods and apparatus for the measurement of the size of particles entrained in a gas. An existing method is to sample the particles from the gas, for subsequent offline analysis. In addition to the difficulty of ensuring that samples are representative of the entrained particulate, it is then necessary to ensure that the particles measured are no more or less agglomerated than those in the gas. The uncertainties associated with these measurements introduce doubts into measurements of the apparent efficiency of gas cleaning devices. It is therefore desirable to measure the concentration and size distribution of the entrained particles directly in situ without sampling. It is also desirable for the measurement device to be capable of being traversed, so as to obtain representative mean values across a large gas duct.
The "size" of a non-spherical particle can be defined in a variety of different ways. For gas cleaning operations, the parameter of most relevance is usually the Stokes diameter, which describes both the intertial and settling behavior of a particle. It is possible, and sometimes preferable, to infer the Stokes diameter from simple off-line measurements such as are obtained by Coulter Counter or image analysis. The Coulter Counter, for example, measures directly the particle volume and, for wide ranges of particle shape, the volume-equivalent diameter is close to the Stokes diameter. However, these indirect measurements are always time-consuming, demand redispersion in a liquid, and require at least two different methods to be used on the same sample in order to infer the precise relationship between the volume-equivalent and Stokes diameters.
Of the devices currently available for direct measurement of Stokes diameter, horizontal, vertical and centrifugal settling devices are of considerable interest for calibration of other aerodynamic instruments. However, they are essentially laboratory techniques which operate on batch samples and which require experience, care and patience to obtain reliable results. Inertial impaction is probably the commonest industrial technique for measurement of Stokes diameter. However, in addition to requiring the gas to be sampled, inertial impaction measurements are subject to uncertainties arising from the possibilities that particles may bounce or blow off from the impaction plates and that agglomerates may break up on impact. Cyclones with sharp cut-off have been used for particle size measurement. Recently an automated 3-cyclone train has been described using an oscillating microbalance to indicate the masses of particles collected. Although this device is automated, it remains a sampling instrument which gives readings in a few wide size ranges at intervals of tens of seconds.
The TSI "Aerodynamic Particle Sizer" or APS is another relatively new instrument which differs from the other devices in measuring the inertial behavior of individual particles. In principle, it can give finer resolution of the particle size distribution than a device such as a cascade impactor which sorts particles into a few discrete bands. Dilute particle-laden gas is accelerated through a nozzle, so that the entrained particles reach a velocity significantly less than that of the gas. Thus, for each particle, the slip velocity relative to the gas depends upon its inertia, i.e. upon its aerodynamic diameter. The velocities of particles issuing from the nozzle were measured by laser-Doppler anemometry, but the commercial instrument measures the time-of flight between two laser beams separated by 120 m. The particle size distribution is built up from measurements of individual particles. In theory this technique should be absolute, but in practice it is necessary to calibrate the instrument using particles of known aerodynamic diameter. While the APS appears to give reliable measurements for spherical particles, there is some evidence that it undersizes irregular particles. Particle orientation effects are insufficient to explain the discrepancy; A contributory cause may be shape features which induce boundary layer separation at relatively low Reynolds numbers and hence have a strong effect on drag. The APS also has the disadvantage that it is another sampling instrument, with a low sample gas rate which would have to be cooled before being passed to the instrument. Like all single-particle counters, it is subject to "coincidence errors", arising from particles whose times-of-flight overlap, so that most gases of industrial interest also have to be diluted before analysis.